1. Field of the Invention
This invention relates to electronic regulation circuits and particularly to a series voltage regulator with low dropout voltage.
2. Description of the Related Art
“Series” low DropOut (LDO) voltage regulators are frequently used for making battery powered circuits. Apart from their regulation function, they also switch unused electronic sub-circuits when necessary so as to reduce electrical consumption of the equipment. They are used jointly with switching power supplies to increase rejection of disturbances emitted by these same power supplies.
These circuits are used to make many mobile telephones on the market. Large efforts are made to increase the performances of these regulators, particularly in terms of load rejection and response to load variations. It is desirable to be able to create an output voltage with a precision of better than one percent, even for particularly low power supply voltages (less than 2 Volts).
LDO type regulation circuits are already known:
FIG. 1 shows an example of a first known system like that described in the publication “Optimized Frequency-Shaping Circuit Topologies for LDOs” by Gabriel A. Rincàon-Mora and Philip E. Allen, IEEE TRANSACTIONS ON CIRCUIT AND SYSTEMS-II: ANALOG AND DIGITAL PROCESSING No. 6, June 1998. This first circuit is based on a cascade with a differential amplifier 110 acting as error amplifier, a follower stage 120 (or inverter) and a PMOS transistor (in the example illustrated) 130 used for voltage regulation supplying a load 141-143. A counter-reaction chain materialized by resistive elements 170 and 180 is used for regulating the power supply voltage at the terminals of the drain of transistor 130. The differential amplifier 110 is loaded by a capacitor 150 making an order zero (0) pole, which gives a high gain in open loop.
FIG. 2 illustrates a second known circuit described particularly in document “A Capacitor-Free CMOS Low-dropout Regulator with Dampling-Factor-Control Frequency Compensation”, Ka Nang Leung and Philip K., T. Mok. IEEE JOURNAL OF SOLID STATE CIRCUIT, VOL. 38, No. 10, October 2003. For reasons of clarity, elements functionally identical to the first circuit have the same references. A regulator based on a chain comprising a differential error amplifier 110, a follower or inverter stage 120, and the PMOS transistor 130 are once again present. As above, the error amplifier 110 is loaded by a capacitive load formed by the capacitors Cm1, CM2 and the DFC amplifier making a capacitor amplifier. The result is once again an order zero pole giving a high gain in open loop.
These two circuits, and in general circuits known according to the state of the art, introduce stability problems that are solved using appropriate pole splitting techniques. These techniques induce large transient responses during sudden current variations due to the large number of poles (in the Nyquist sense) in the slaving chain.
The result is loss of precision achieved in the regulator output voltage.
It is desirable to improve known regulation circuits, particularly for response transients to sudden variations in the current demand by the load.